Apparatus for biological processing of metal-containing ores

ABSTRACT

An apparatus for use in bioleach processing of metal-bearing solids is disclosed. The apparatus includes a containment means adapted for containing a slurry made up of metal-bearing solids, a predetermined quantity of water, oxygen, carbon dioxide, nutrients and a species of microorganisms capable of oxidizing some portion of the metal-bearing solids and obtaining energy for growth from that oxidation. The apparatus further includes a plurality of horizontally oriented porous, flexible membrane diffusers adapted for introducing oxygen into the bottom of the reactor vessel in the form of small widely dispersed bubbles.

RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No.005,670, filed Jan. 21, 1987, which is a continuation-in-part ofapplication Ser. No. 827,324, filed Feb. 7, 1986, now issued as U.S.Pat. No. 4,732,608 and application Ser. No. 884,204 filed July 10, 1986,now issued as U.S. Pat. No. 4,728,082, the disclosures of which arehereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field

This invention relates to a process and attendant apparatus for use inprocessing metal-containing ores by use of a biological (hereinafter"bioleaching") technique. More particularly, this invention is directedtoward a process and apparatus for use in processing preciousmetal-bearing pyrite ore concentrates which are not efficientlyleachable by conventional processes and means, such as leaching usingcyanide solutions.

2. State of the Art

Recent interest in the metallurgical field has focused on the use ofspecial types of autotrophic bacteria, e.g. thiobacillus ferrooxidansand thiobacillus thiooxidans, in treating sulfide ores and concentrates.The use of such bacteria in heap leaching treatments to solubilizecopper from low-grade ores has been known for several decades.

Currently, however, the interest in applying this biochemical technologyhas been focused on continuous processes to treat sulfide concentrates.These continuous processes either make the concentrates more susceptibleto conventional cyanide leaching or actually extract the desired metalfrom the concentrate.

Particular attention has been focused on gold-bearing, silver-bearing,or platinum-bearing pyrites and arsenopyrites that are, at best,marginally susceptible to cyanide solution leaching. These concentrates'insusceptibility to cyanide leaching is due to the desired metals, e.g.gold or silver, being encapsulated by the pyrite crystal. The pyritecrystal is insufficiently porous to allow penetration of the cyanidesolution for a metal-cyanide dissolution reaction to take place.Comminution of the metal-bearing pyrite, in itself, does not exposesufficient metal values to be economically feasible inasmuch as greatlyincreased cyanide solution and energy consumption are required. Theabove-described bacteria can, however, induce the biooxidation ofsulfide and iron in the unsolubilized pyrite crystal, leaving the gold,silver or platinum intact. The resulting residue, after separation ofthe soluble biooxidation products, is amenable to metal extractionemploying conventional cyanide, thiourea, or thiosulfate solutionleaching techniques. On occasion, even a partial biooxidation of themetal-bearing pyrite by the above-described bacteria is sufficient toallow successful cyanide solution leaching of the resulting residue.

The described process is adaptable to the leaching of other metals. Forexample, chalcopyrite can be leached for its copper content, and zincsulfides can be leached to produce zinc sulfate solutions (ZnSO₄). Otherelements present as sulfides may also be solubilized, such as antimonyand arsenic.

The current processes using the above-described bacteria forsolubilizing the metal-bearing sulfide ores and concentrates are veryenergy intensive. The chemical reaction used by these bacteria isoxidation. Hence, oxygen transfer is a key step in the process.

Approximately an equal weight of oxygen is required to oxidize pyrite.The systems currently employed in the art require one horsepower hourper approximately 2.5 to 4 pounds of oxygen transferred into liquidphase.

Consequently, to oxidize one ton (2,000 pounds) of concentrate, thesesystems consume approximately 400 to 600 kilowatt hours (KWH) of energy.

Metallurgical processing by leaching typically employs a number of tanksoperating in series, each tank overflowing or cascading into asubsequent tank. The total retention time in the circuit (i.e., theseries of tanks) is that required for processing. Reagents required forleaching are usually added to the first tank, and if necessary, tosubsequent tanks. With bioleaching, there is a significant time requiredfor bacterial growth to reach a level of suitable bioactivity. Simplyadding bacteria to the first tank will not immediately providesufficient numbers of microorganisms to achieve any great degree ofprocessing. Furthermore, as the pulp flows from one tank to the next andthe bioreaction continues, the amount of soluble by-product materialproduced can become very high. Soluble by-product material, e.g., metalsulphates, sulfuric acid, and arsenic acid, is a product of thebioleaching operation, which if present in the reaction tank inexcessive proportion inhibits the speed of the reaction. Thus, withoutselective removal of this soluble by-product material, the reaction rateis diminished and the process is slowed.

One of the critical problems involved in developing a workable processis the transfer of nutrients and oxygen into the tanks in sufficientquantities so as to be readily assimilated by the bacteria. The bacteriarequire a supply of nitrogen, potassium, phosphorus and carbon dioxideas nutrients. These nutrients are typically provided by adding ammoniumsulphate, potassium, phosphates and gaseous carbon dioxide to the tanks.Problems associated with transfer of the oxygen are distinguishable fromthose encountered providing nutrients and carbon dioxide. Since oxygentransfer is critical and the quantity required is very large, this partof the process is of paramount importance to overall process cost andperformance. The method practiced conventionally involves injectinglarge quantities of oxygen directly into the solution and providing amixing means whereby the oxygen is dispersed or distributed within thesolution. These processes involve introducing the oxygen andtransferring it from a gas phase into an aqueous phase, i.e., dissolvingit within the solution.

The method conventionally adopted to effect this transition typicallyutilizes turbines which are placed within the slurry and rotated at highspeeds. Though the turbine action does provide considerable mixingaction, i.e., dispersion the oxygen, within the solution; the rotationof the turbines also produces cavitation effects. These effects causethe air bubbles within the solution to be forcedly aggregated intolarger air masses or bubbles due to the vacuum effects and turbulenceattendant the action of the turbine blades. Resultingly, the turbines,though functioning to disperse the air within the slurry, also functionto create large air masses or bubbles which have a relatively smallsurface area to volume ratio. A basic problem confronting theconventional technology is the power requirement requisite to operatethe turbines. The turbine power is that required to turn the blades at asufficient velocity to achieve the desired quantity of oxygen beingintroduced into the aqueous phase of the solution. Oxygen in this phasemay be readily assimilated by the bacteria. A considerable mixing actionis required, necessitating a high tip speed on the turbine rotor blades.Understandably, this high tip speed is only obtained by an infusion ofconsiderable quantities of energy into the turbine itself.

A second problem confronting the current technology is the removal ofsoluble by-product matter produced within the solution by the reactionseffected or initiated by the presence of the bacteria. One typicalapproach to this problem is the use of a thickener. The slurry isadmitted into the thickener and soluble components are removed via theoverflow of the slurry/thickener mixture. This approach generallyresults in the bacteria, which are suspended within the liquid phase,being carried away together with the soluble matter, in the overflow.This removal of bacteria from the slurry slows the process reactionrate. Furthermore, the slurry thickener mixture is not aerated duringthe separation of the soluble material from the slurry. Therefore, thebacteria which remain with the solids are deprived of requisite oxygenand resultingly tend to slow their activity and further delimit the rateof the process.

A third major problem of the conventional process is the length ofoverall retention time required to achieve a desired extent ofbiooxidation. Systems currently employed require a retention time ofmany days. The retention time is inversely proportional to reactionrate, which is found to be enhanced by maximization of oxygen andnutrient supply. The reaction rate is delimited by the presence ofreacted products and by-products in the reactor vessel and by the lossof biomass (i.e., microorganisms or bacteria) to the reactor effluent.

Failure of the current art to address effectively the above aspects ofbioleaching has resulted in current bioreactors and processes beingmarginally efficient in both cost and process performance.

SUMMARY OF THE INVENTION

The reactor of the invention includes a containment means adapted forcontaining a slurry of metal-bearing solids. The containment meansincludes a bottom surface and an upstanding wall mounted in that bottomsurface. An oxygen supply means, for introducing an oxygen-containinggas is mounted within the containment means. The supply means includes aplurality of diffusers which individually include a porous, flexiblefabric membrane diffuser surface adapted to receive a supply of oxygencontaining gas and introduce that gas in the form of bubbles into theslurry within the containment means. Each diffuser is positioned suchthat the face thereof is oriented substantially in a horizontal plane.Various diffuser configurations are disclosed. A preferred embodimentincludes a planar diffuser face having a circular shaped perimeter. Asecond embodiment defines a planar diffuser face having a quadrilateralperimeter. A third embodiment utilizes a plurality of elongate tubularshaped diffusers arranged such that their respective longitudinal axesare oriented parallel to the horizon.

Various structural arrangements are contemplated for mounting thediffusers in the containment means. In one embodiment, a stationaryframework is mounted within containment means to provide a fixedplatform in which to mount the diffusers. In a second embodiment, thediffusers are mounted to a rotatable platform. In one construction, thisrotatable platform includes an upright drive shaft located centrallywithin the containment means. A plurality of radially extending arms aremounted to the shaft. Individual diffusers are spacedly mounted on thearms. The shaft and its attendant arms is made rotatable about avertical axis whereby gas dispersed from the plurality of diffusers isintroduced into the slurry in a generally helix-shaped pattern. A gastransfer means for conveying pressurized gas (e.g., air) from anexternal supply to the individual diffusers is also contemplated withinthe scope of the invention.

A sweeping means, adapted for sweeping and otherwise scouring the bottomof the containment means may also be included in the invention. In thoseembodiments wherein the diffusers are mounted to a stationary platform,the sweeping means is especially important in that it operates to breakup concentrations of solids which have settled out of the slurry ontothe bottom of the containment means and thereby assists in precludingthose concentrations from building up to the point where they cover overthe stationary diffusers and hinder their proper operation.

An air lift suspension means may also be mounted on the containmentmeans. The suspension means may be associated with the sweeping meanswhereby the suspension means may be used to resuspend the solids whichhave settled out of the slurry.

Whereas previous bioleach reactors have relied upon turbines and similarmechanical means to agitate the slurry and thereby retain the solids insuspension, the present reactor relies in large part on the agitationproduced by the air/gas bubbles rising through the slurry to retainthose solids in suspension. Given a sufficient diffuser area andquantity of air being introduced over that diffuser area, the inventionachieves not only the requisite quantity of oxygen introduction into theslurry in a form which is assimilatable by the bacteria, but furthermoreachieves a sufficient agitation of the slurry to retain the solids insuspension.

The air supply means of the invention generally involves theintroduction of minute air bubbles near the bottom regions of the tankby a plurality of horizontally oriented membrane diffusers. In thoseembodiments wherein the diffusers are rotated within the containmentmeans, the diffusers are configured to have a generally streamlinedshape which may pass through the slurry with minimal drag and create anominal amount of agitation and turbulence within the slurry. Inpreferred constructions, the diffusers are thin planar panels. Thenarrow width of the panel is directed into the slurry as the diffuser isrotated. In other words, the thin width of the panel constitutes theprojected area or silhouette area for purposes of evaluating the drag onthe diffuser. The diffusers are mounted on the arms and oriented tominimize any drag force on the diffuser as the arm rotates and drivesthe diffuser through the slurry. As the diffuser rotates, the slurry inclose proximity to the diffuser flows over the membrane face of thediffuser. This slurry flow speed is sufficient that the particulates andliquid of the slurry act to scour and cleanse the slurry-exposedmembrane face of the diffuser. This scouring and cleansing actionreduces the tendency of the pores in the diffuser membrane face to plug.A plurality of diffusers may be mounted in spaced relationship along thelength of each radial arm of the slurry mixer mechanism. The radial armsmay be rotated about an essentially upright, vertical axis. Thediffusers are thus rotated so as to distribute rising air bubbleseffectively over a substantially horizontally oriented planar area ofthe lower regions of the tank. The arms are rotated at a fairly slowspeed whereby each diffuser produces a generally spiral helixconfiguration of bubbles which rise through the slurry in the tank.

The number of individual diffusers employed and their location relativeto each other are determined by the total amount of air required by thebiooxidation occurring within the bioreactor. Further, the number andlocation of diffusers are determined by the oxygen transfer efficiencyand capacity of the individual diffusers.

The diffusers may each include a horizontally mounted frame having aporous membrane fitted thereto. This membrane may be held in asubstantially horizontal orientation by its mounting frame. The membraneincludes a plurality of pores or orifices oriented such that theapparent air flow through these pores or orifices is outwardly throughthe membrane of the diffuser and substantially perpendicular to theslurry flow over the diffuser membrane surface. The diffusers aremounted on the radial arms of the slurry mixer so as to benefit from anylocal turbulence and cleansing action of the slurry in close proximityto the diffuser which is generated as the diffuser passes through theslurry. The pore size of the diffusers and the location of diffusermountings on the radial arms of the slurry mixer mechanism aredetermined to produce optimally air bubbles having an approximate meandiameter of 4.5 millimeters or less. It is recognized that the finer thebubbles produced, the more readily is the oxygen contained therein,assimilatable by the bacteria.

The air supply means of the instant invention functions to achieve anenhanced surface area to volume ratio of the air bubbles introduced intothe slurry. At the same, the supply means minimizes the opportunity andprobability of aggregation of the various bubbles into larger masses ofbubbles having a smaller surface area to volume ratio. In this manner,the instant invention achieves a greater assimilation condition orprobability for the oxygen to be transferred into liquid solution ordirectly to the bacteria for purposes of assimilation and subsequentconsumption in the biooxidation reaction. Further, the rotation of theradial arms effects a dispersion of the bubbles through the slurry witha minimal agitation of the slurry within the bioreactor vessel. Thisradial arm rotation methodology minimizes the energy consumption of thesystem as compared to the conventional turbine equipped systems. Byminimizing the agitation the instant air supply means promotes theretention of the bacteria in contact with the suspended solids andthereby maintains the reaction rate.

The center shaft may be a large hollow pipe fitted with internal pipingnecessary to provide air to the radially-mounted mixer arms on which aremounted the diffusers. The selection of a hollow pipe permits theintroduction of air from a supply located external to the tank. Thehollow pipe is typically mounted with a lower, open end whichcommunicates with the slurry. By this construction the slurry risesthrough the interior of the pipe, thereby surrounding the internalpiping within the hollow pipe. Air may be injected into the hollow pipethrough the internal piping and be channeled downwardly eventually beingdriven to the diffusers through the radial mixer arms positionedproximate the bottom of the tank.

Alternately, a solid center shaft may be employed. This alternativeconstruction may include having the center shaft mounted on a foot orthrust bearing. An air conveying pipeline may be extended into a recesswell defined within the portion of the shaft proximate its seatingwithin the foot bearing. The recess well communicates with the mixerarms and the diffusers mounted thereon. The recess well includes asealing means configured to retain air received within the well fromescaping outwardly into the slurry except by passage through the mixerarms and their associated diffusers.

Additional internal piping may be provided in the hollow pipe forpurposes of circulating fluids along the height of that pipe. This fluidcirculation is directed toward the removal of heat generated within thereaction vessel, by the oxidation reactions occurring therein. Theadditional internal piping forms a heat exchanger by which cold fluidsmay be circulated through pipes whose external surfaces are in contactwith the heat bearing slurry. Heat from the slurry is transferredthrough the walls of the piping and is thereafter transferred to thecirculating cold fluid. Upon receiving that heat, the now heated fluidis directed away from the vessel to a disposal site.

Slurry may also be circulated from the lower portions of the tank,through the center shaft pipe, to radially-mounted riffle tubes at theupper end i.e., top of the tank. These riffle tubes may be used toenhance gravitation separation of high specific gravity solids, i.e.free gold, before the circulated slurry is returned to the general bodyof slurry in the tank. Slurry circulation across the riffle tubes is anenergy efficient means of collecting free gold and other high specificgravity solids or deposits. The riffle tubes operate to prevent theaccumulation of such solids or deposits on the bottom of the bioreactortank. The riffle tubes are mounted to rotate with the center shaft aboutthe central longitudinal axis of the reactor vessel. Each of the riffletubes is fitted with a discharge port or spout for discharging thecirculated slurry from the tube outward and onto the upper surface ofthe body of slurry. The discharge port is positioned above anycontemplated slurry level. As a result the slurry being discharged fromthe tubes always falls downward onto the body of slurry. This fallingmotion together with the rotation of the tubes provides a downward flowof slurry which is distributed over a large portion of the upper surfaceof the body of slurry. This flow has the effect of suppressing anybuildup of foam on the upper surface of the body of slurry.

In some constructions, the radial mixing arms are mounted to a collarwhich is slidably mounted on the center shaft. This collar, togetherwith its attendant arms, is made slidable along the height of the centershaft. A lifting mechanism to mechanically raise and lower the radialmixing arms of the bioreactor may be provided to facilitate the cleaningof the mixing arms and the diffusers mounted thereon.

A vacuum filter may be mounted within the bioreactor tank to removeclear liquor containing dissolved products and by-products e.g. sulfuricacids and assorted salts from the vessel while leaving the bacteria andslurry solids within the vessel. Generally, this filtering involves thecontinuous or semi-continuous removal of a quantity of slurry from thegeneral body of slurry. The unoxidized solids within this quantity ofslurry are separated out by use of a cyclone. The unoxidized solids arethen returned to the reactor vessel. The partially oxidized solids areseparated from the slurry liquor and are directed to a second reactorvessel and subjected to further oxidation. The soluble by-products ofthe oxidation process, e.g. sulfuric acid and assorted salts, aredirected to waste.

The process of the instant invention generally includes the steps ofgrinding the concentrate or ore; placing the concentrate or ore andother reactants, including a form of bacteria capable of oxidizingsulfide solids, e.g. thiobacillus ferrooxidans or thiobacillusthiooxidans in a primary bioreactor in such a way as to achieve a highrate of bioreaction; removing the soluble products, by-products andpartially reacted solids; and directing the insoluble and partiallyreacted solids to a secondary bioreactor or series of bioreactors toallow for the completion of biooxidation, while returning any unoxidizedsolids to the primary bioreactor vessel.

Optimization of the overall biooxidation rate, thus minimizing solidsresidence time, equipment size, and cost, can be achieved only if theprimary bioreactor is operated in such a manner so as to achieve themaximum consumption of oxygen, e.g. biooxidation rate, withoutattempting to control coincidentally the extent to which the concentrateor ore constituents are being oxidized while within the primarybioreactor.

The normally recognized biochemical oxidation reaction involves thedissolution of oxygen in water, followed by the bacterial assimilationof that dissolved oxygen. The bacteria e.g. thiobacillus thiooxidans orthiobacillus ferrooxidans, subsequently use the assimilated oxygen tooxidize biochemically the sulfide and iron species. The bacteria obtainsenergy for growth from oxidizing these species. In order for thebacteria to oxidize the iron it must be in a bivalent form (Fe++), i.e.the ferrous form. The bacteria converts the iron to trivalent form(Fe+++), i.e., the ferric form.

The bacteria may also oxidize a variety of sulfides, e.g. thiosulfateion (S₂ O₃ --); the tetrathionate ion (S₄ O₆ --); soluble sulfides, i.e.those containing the sulfur ion S--; insoluble sulfides; and elementalsulphur. The end result is the production of a sulphate ion (SO₄ --).This biooxidation is the essence of the bioleaching process.

In a bioreactor such as that of the instant invention, the bioreactionenvironment can be controlled such that oxygen transfer is alsoaccomplished interfacially from a gas directly to the bacteria. In otherwords, oxygen transfer is effected without involving the otherwisereaction rate-limiting, oxygen dissolution step.

The secondary bioreactor or bioreactors of the process of the instantinvention generally operate(s) at oxygen uptake rates similar to thebioreactors of the current art. However, since as much as 90% of thebiooxidation occurs in the primary bioreactor, it is the primarybioreactor which is rate limiting. Thus, due to the enhanced efficiencyof the primary bioreactor of the instant invention, the secondarybioreactor and the overall process requires much less time to achieve adesired extent of biooxidation.

The essence of the process of the instant invention is the control ofthe reaction environment within the bioreactors, particularly theprimary reactor. The factors controlled in each bioreactor in theprocess of the instant invention include temperature, the rate andmechanism of oxygen input, the ratio of biomass (i.e. bacteria) tosuspended solids, the ratio of reacted (i.e. inert) solids to unreactedsolids, the concentration of soluble species generated as products orby-products, and the concentration of carbon dioxide and nutrientsprovided for bacterial growth.

Since the biooxidation reaction produces heat, a mechanism for heatremoval may be provided as part of the process. Oxygen supply in theform of very small bubbles of sufficient number to sustain the bacteriais, if insufficient, the limiting factor on overall rate of the process.Both temperature control and oxygen supply are factors governed by themechanical design of the bioreactor.

Maintaining the optimum ratio of biomass to reacted solids and theoptimum ratio of reacted solids to unreacted solids is a task requiringthe use of equipment ancillary to the bioreactor. A quantity of slurryis continuously or semi-continuously withdrawn from the reactor vesseland processed by this ancillary equipment. Reacted and partiallyoxidized solids can be separated from unreacted, unoxidized solids bythe use of a selective centrifugal force separation in a cyclone,centrifuge, or alternatively a gravity settling device (e.g., ahydro-separator). The separation employs the differences in particlesize or relative density, i.e. specific gravity, of the feed and productsolids, allowing the more rapid settling of the larger and more densesulfide, i.e. unreacted unoxidized solids as compared to the less denseoxidized and partially oxidized solids. Selective flocculation oragglomeration of these species may also be employed to enhance theefficiency of their separation.

Soluble by-product constituents in the reaction slurry act to limit therate of bioreaction. The control of soluble by-product constituents inthe reaction slurry may be achieved by the removal of suspendedsolids-free liquor from the slurry while retaining the bacteria withinthe slurry. This is achieved by a filtration mechanism internal to thebioreactor or a separation of liquid and solids by the use of a cyclone,centrifuge, or clarifier. Flocculation of all the suspended solids willenhance the solids-liquid separation. The clarified filtrate or overflowis removed from the system, while all captured solids are recycled tothe bioreactor.

It is desirable for the process of this invention that biomass, i.e.bacteria, be maintained at as high a number level as possible in eachbioreactor. The concentrate or ore feed, oxygen, carbon dioxide, andnutrients provide an environment for bacteria to grow and increase innumber. Whatever bacteria leave the bioreactor thus leave only incombination with product, i.e. reacted, suspended solids, to which theyare physically attached. Mechanisms for the removal of solubleconstituents and liquors are designed so as to not remove bacteriacoincidentally.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevated perspective view of the bioreactor vessel of theinstant invention including a cut-away portion which reveals a centrallypositioned support member fixedly mounted with a plurality of rotating,radially extending arm-like members positioned about the lower regionsof that support member;

FIG. 2 is an elevated perspective view of the lower portion of thesupport member shown in FIG. 1. Two diffuser configurations areillustrated;

FIG. 3 is a cross-sectional view of the bioreactor vessel shown in FIG.1;

FIG. 4 is a cross-sectional view taken of the support member of thebioreactor vessel shown in FIG. 1 taken along sectional lines 4--4;

FIG. 5 is a schematic process diagram illustrating the process of theinstant invention;

FIG. 6 is a partial schematic process diagram illustrating a filteringprocess of the instant invention;

FIG. 7 is a schematic process diagram illustrating a separation processof the instant invention;

FIG. 8 is an elevational perspective view of one diffuser structure ofthe invention;

FIG. 9 is a top view of the diffuser shown in FIG. 8;

FIG. 10 is a side view of the riffle tube arrangement as found in thebioreactor vessel illustrated in FIG. 1;

FIG. 11 is a cross-sectional view of the riffle tube arrangementillustrated in FIG. 10 taken along sectional lines 11--11;

FIG. 12 is a cross-sectional view of the riffle tube arrangement shownin FIG. 11, taken along sectional lines 12--12;

FIG. 13 is an elevated perspective view of a rake-like extension;

FIG. 14 is a top view of the rake-like extension shown in FIG. 13illustrating the positioning of the extension vis-a-vis its support arm;

FIG. 15 is an elevational perspective view of the diffuser and rake-likeextension mountings on a support arm;

FIG. 16 is a perspective view of an alternative diffuser structureadapted for mounting on a radial mixing arm;

FIG. 17 is a top view of the diffuser shown in FIG. 16;

FIG. 18 is a top view of the containment vessel of the inventionillustrating a plurality of the diffuser structures shown in FIGS. 16and 17 rotatably arranged in a radial arrangement within the containmentvessel;

FIG. 19 is an elevated perspective view of an altenative diffuserstructure of the invention;

FIG. 20 is a top plan view of the diffuser structure illustrated in FIG.19;

FIG. 21 is a schematic view of an experimental pilot reactor vessel ofthis invention;

FIG. 22 is a schematic view of an experimental pilot reactor vesselsystem of this invention;

FIG. 23 is a graph illustrating the result of a liquids analysis of theexperimental trial of the apparatus and process of the instantinvention;

FIG. 24 is a graph illustrating the results of a gas analysis of anexperimental trial of the apparatus and process of the instantinvention;

FIG. 25 is a graph illustrating the results of a solids analysis of anexperimental trial of the apparatus and process of the instantinvention;

FIG. 26 is a graph illustrating the sulfide oxidation rate results of anexperimental trial of the apparatus and process of the instantinvention;

FIG. 27 is a graph illustrating the results of a liquids analysis of anexperimental trial of the apparatus and process of the instantinvention;

FIG. 28 is a graph illustrating the results of a gas analysis of anexperimental trial of the apparatus and process of the instantinvention;

FIG. 29 is a graph illustrating the results of a solids analysis of anexperimental trial of the apparatus and process of the instantinvention;

FIG. 30 is a graph illustrating the sulfide oxidation rate of anexperimental trial of the apparatus and process of the instantinvention;

FIG. 31 is a graph illustrating the oxygen concentration and oxygentake-up rate in the primary reactor in an experimental trial of theapparatus and process of the instant invention;

FIG. 32 is a graph illustrating the oxygen concentration (actual valuedivided by 10) and the oxygen take-up rate (actual value multiplied by2) of the secondary reactor vessel of an experimental trial of theapparatus and process of the invention;

FIG. 33 is a graph illustrating the desired relationship of oxygenconcentration and sulfide oxidation rate in a reactor vessel of theinvention;

FIG. 34 is a graph illustrating an undesired relationship of oxygenconcentration and sulfide oxidation rate in a reactor vessel of theinvention;

FIG. 35 is a graph illustrating the efficiency of oxygen transfer ratefor a diffuser of this invention;

FIG. 36 is a side view of an alternative air supply means for thereactor vessel of this invention;

FIG. 37 is a perspective view of a third diffuser structure of theinstant invention;

FIG. 38 is a cross-sectional side view of the diffuser shown in FIG. 37;and

FIG. 39 is an elevated perspective view of a second embodiment of thebioreactor vessel of the instant invention including a cut-away portionillustrating a stationary grid-like frame having a plurality ofhorizontally oriented diffusers positioned within a containment vessel.A plurality of radially extending arms having rakes mounted thereto isrotatably positioned below the grid-like frame;

FIG. 40 is a top plan view of the containment vessel shown in FIG. 39illustrating the stationary diffuser fited grid framework in associationwith the rotatable rake-fitted radial arm arrangement.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

As shown in FIG. 1, a bioreactor vessel, generally 13, of the instantinvention includes an open-ended tank 14 having suspended therein an airsupply means generally 15 adapted to inject air received from a sourceexterior of the tank 14 into a liquid medium, generally 16, which iscontained within the tank.

The tank 14 consists generally of a bottom member 17 which is fixedlyand sealingly mounted with an upstanding vertical sidewall or sidewalls18. As shown, the bottom member 17 may be substantially planar andcircular in plan view. The upstanding vertical walls 18 may be a singletubular-shaped wall, whereby the tank obtains a substantiallycylindrical configuration having an open port or end 20. In a preferredembodiment, the vertical walls 18 define a tank diameter 21 whichremains constant over the height of the tank. The upright walls 18 andthe bottom planar member 17 are fabricated from materials which arechemically resistant to the solids, slurry or by-products which may behoused within the tank. Materials such as stainless steel are generallyused in constructing the tank. The height of walls 18 is preferably ofsufficient dimension to permit a storage of a fluid (slurry), withintank 14, having a depth of at least twelve (12) feet.

Positioned within the tank is an air supply means 15. As shown, thissupply means may include an elongate tubular support shaft 22 which maybe positioned centrally and upright vertically within the tank 14. Shaft22 includes a longitudinal axis 24 which is oriented substantiallyupright and which furthermore passes through, or may be co-linear to,the longitudinal axis 26 of the tank 14. The support shaft 22 may bestructurally configured in a variety of shapes. As shown, the supportmeans may be a substantially cylindrical, hollow tubular pipe memberwhich extends from an elevation which is above any anticipated liquidlevel 27, to be obtained within the tank 14, downwardly to an elevationproximate the bottom plate 17 of the tank.

Support shaft 22, as shown in FIG. 4, may include an exterior wall 30and an interior wall 32. Interior wall 32 defines an interiorcylindrical channel 34. Channel 34 provides a location for thepositioning of a plurality of cylindrical tubular pipes, generally 35.

The oxidation and reduction processes accomplished within the reactorvessel are exothermic in nature. The instant invention contemplates aheat transfer mechanism whereby heat produced within the slurrycontained in the reactor vessel may be dissipated or removed from thereactor vessel.

As shown, in a preferred construction a second tubular, cylindrical pipemember 43 is positioned within shaft 22. Tubular pipe member 43 issubstantially similar in configuration to shaft 22 and further sharesthe same longitudinal axis 24. A first channel 49 is defined by theinterior wall 32 of shaft 22 and the exterior face 47 of tubular pipemember 43. Channel 49 is substantially annular in cross-section. Channel49 extends along the height of shaft 22. Channel 49 communicates with asupply means 51 positioned on the upper end 41 of shaft 22. Supply means51 is adapted to supply a pressurized flow of fluid to channel 49.

Channel 49 receives that fluid and directs it downwardly along thelength of the channel 49.

The fluid, e.g. water, flows along the surface interior walls 32 andacts to absorb heat from shaft 22 and pipe member 43. The described heatis that which is generated within the container-retained slurry 16 dueto the oxidation reactions occurring therein. Upon the fluid reachingthe end of channel 49, proximate the bottom 17 of the tank, the fluid isdirected into a channel 53 defined by a tubular pipe member 55 housedwithin shaft 22. The heat-laden fluid, being under pressure, isthereafter driven upwardly through channel 53 until reaching a locationproximate the upper reaches of shaft 22. Since the slurry within channel34 contacts the exterior surface of pipe 55, the fluid in channel 53also absorbs heat from slurry within the channel 34. A discharge means(not shown) is connected to tubular pipe member 55 and operates toreceive the heat-laden fluid from the channel 49 and dispose of same.The arrangement of channel 49, together with pipe 55, supply means 51,and the discharge means, forms the heat exchanger adapted to remove heatgenerated by the exothermic reactions occurring within the tank 14. Heatmay also be removed from the slurry by the air introduced into thevessel by air supply means 15, i.e., the air injected into the slurry 16may be of a sufficiently low temperature and humidity that as it risesthrough the slurry, after its introduction therein, it absorbs heat fromthe slurry and conveys that heat upwardly eventually discharging it tothe environment upon the air bubble breaking the surface of the slurry.Alternately, the heat exchanger may include fluid conducting coilspositioned within the reactor vessel, e.g. about the walls 18 of thereactor vessel. Heat is removed from the vessel so as to maintain aslurry temperature within the range of 30° C. to 36° C.

A pipe 57 positioned within channel 34 of shaft 22 defines an interiorchannel 59. Channel 59 is used to receive a pressurized supply of air oroxygen-containing gas from a source (not shown) exterior to the tank 14.The channel 59 directs that air down to the lower regions of the supportshaft 22. The air is thereafter introduced into the slurry extant withinchannel 34 for purposes of air lift suspension of the slurry and theparticulates (solids) within the slurry itself.

As shown in FIG. 10, pipe 57 extends to a location proximate the bottom17 of tank 14. The pipe 57 may be fitted on its end with a diffuser 60.Diffuser 60 may be constructed from a rigid frame having a porous,flexible fabric stretched over that frame. Air is received, underpressure within a hollow chamber defined within the diffuser and drivenoutward through the diffuser faric into the slurry. As illustrated, airis injected through diffuser 60 into the channel 34. Channel 34communicates with the slurry 16 within the tank 14 by means of an accessport 61. Access port 61 is defined by the open end 62 of shaft 22 whichis positioned above and out of contact with bottom member 17 of tank 14.As air is injected into the interior of channel 34, a portion 64 of theslurry 16 contained within the channel 34 is driven upwardly alongchannel 34. Upon reaching the region 66 of channel 34, the slurryportion 64 is directed through a plurality of channels 68, i.e.,channels 68 communicate with channel 34. Each channel 68 is definedrespectively by a hollow riffle tube 70 which is fixedly mounted toshaft 22.

As shown in FIG. 10, each riffle tube 70 includes a substantiallycylindrical housing 72 which extends radially from shaft 22. Each riffletube 70 is fitted at its end 74 with a discharge port 76. The slurryproceeds along the length of tube 70 and is eventually discharged fromtube 70 through port 76. Port 76 directs the slurry downward. Theslurry, in being discharged from the riffle tubes 70, impacts againstthe upper surface 78 of the slurry 16 contained within the tank 14. Thisimpact or discharge of the riffle tube-contained slurry functions tobreak down foam formations produced on the slurry surface 78 due to thereactions and turbulence within the slurry 16. Since the riffle tubesare rotated about the axis 24 of shaft 22, the riffle tubes operate todischarge air lifted slurry over a substantially circular pattern aboutthe surface 78 of slurry 16.

The length of each of the riffle tubes 70 may be varied such that theplurality of tubes provides a series of concentric circular dischargepaths over the surface of the slurry. In other words, each of the riffletubes may be dimensioned to discharge slurry along a unique anddistinctive circular path on the upper surface of the body of slurry,i.e. each path has a distinctive and unique radius.

Each riffle tube 70 is fitted with a sawtooth floor structure 80 mountedon the interior wall 82 of the tube 70, i.e., on the floor 83 of theinterior of tube 70. These sawtooth structures function to trap solids,e.g. free gold, silver, or other precious metals having high specificgravities, e.g., above six (6), which are contained within the slurryflow being directed through the riffle tubes 70. The principles madeoperative in the use of these sawtooth structures are essentially thoseemployed in a conventional sluicing operation.

As shown in FIG. 11, the upper portion of each riffle tube 70 is fittedwith a manually openable hinged door 71 adapted for permitting the userto access the channel-housed sawtooth structures 80 for purposes ofremoving the trapped solids.

As shown in FIG. 4, within channel 34 are positioned a pair ofcylindrical, tubular pipe members 84 defining a pair of interiorchannels 86. In an alternate construction as shown in FIG. 36 thesepipes 84 and channels 86 may be external to shaft 22.

Pipes 84 extend from a supply means 88 positioned proximate the end 87of shaft 22 downwardly through channel 34 to a location proximate thelocation of a plurality of radially extending arms 90. Supply means 88is adapted to provide a supply of oxygen-containing gas, under pressure,to channels 86 and force that gas along the length of those channels 86.Channels 86 communicate at their ends with one or more channels 91defined, respectively, within the interior of each radially extendingarm 90.

Each radial arm 90 extends essentially perpendicular outwardly i.e.radially, from the support shaft 22 and is dimensioned to extend to alocation proximate the wall 18 of the tank 14. Each radial arm 90 may besupported by a support arm 91 which extends from the shaft 22 outwardlyand is fixedly mounted on the radial arm 90.

As shown in FIG. 1, each radial arm 90 is fitted with a plurality ofdiffusers 92 which communicate with the interior channel 94. Air isdriven downward through channel 86 and directed into the interiorchannel 94 housed within each radial arm 90. Thereafter, the air isdirected through the diffusers 92, thereby permitting the oxygen or airto be directed upwardly and outwardly into the slurry 16 residing withinthe tank 14.

In preferred embodiments, the diffusers 92 each include a permeable,replaceable membrane 93 having a hydrophobic outer surface. The membrane93 defines a plurality of extremely small pores or orifices thereinpreferably having mean diameters of ten (10) microns or less. In apreferred construction, the membrane 93 is fabricated from a nylon,polypropylene, or polyester fabric having a sealant film, e.g., urethaneacrylic copolymer or polytetrafluoroethylene, applied or laminatedthereon. Suitable membrane materials may include those availablecommercially under such trademarks as GORETEX, TYVEK, VERSAPOR andENTRANT. Alternatively, a porous fabric manufactured by Gummi Jager ofWest Germany may also be used. The membrane 93 is held within a rigidframe structure 94 which retains the membrane 93 in a selectedrelationship with its respective rotating arm 90.

In the embodiment of FIG. 1, each diffuser 92 defines a substantiallyplanar quadrilaterally perimetered diffuser surface which is orientedgenerally horizontally. Each diffuser 92 of this configuration is shownmore detailedly in FIGS. 19 and 20. Each diffuser includes a frame 94which retains a quadrilaterally shaped flexible membrane 93. The frame94 is mounted to a air inlet conduit 95 which communicates with a hollowair conveying channel 96 defined within the radial arm 90. As shown inFIG. 1, the diffusers 92 are arranged such that the longitudinal axis 97of each diffuser 92 is oriented parallel to the longitudinal axis 98 ofits respective radial arm 90. This particular orientation is only one ofmany possible orientations. The principal requisite of the invention isthe orientation of the diffuser surface, as defined by the membrane 93,in a horizontal orientation. Whether the longitudinal axis of aquadrilaterally configured diffuser surface is oriented parallel,perpendicular or otherwise transverse of the longitudinal axis of theradial arm 90 is not of serious consequence.

FIGS. 2, 8 and 9 illustrate a second embodiment of a diffuser, generally92A. As shown, the frame 94A is configured to retain a circular shapedflexible membrane 93A. The frame 94A is mounted to an air inlet conduit95A which in turn is mounted as an communicates with the air conveyingchannel 96A of radial arm 90.

FIGS. 16-18 illustrate a third diffuser embodiment generally 92B. Inthis construction, each diffuser 92B is formed in the shape of a rightcylinder. A porous, flexible membrane 93B forms the sidewall of eachcylinder. The ends of the cylinder may be formed by the support frame.Alternatively, they also may be covered by the porous flexible membrane93B. The membrane 93B is retained in place by a skeletal rigid frame94B.

As shown in FIGS. 16-18, a plurality of diffusers 92B, each having alongitudinal axis 97B, are positioned side by side, the respective axis97B of each diffuser 92B being oriented parallel to the other diffusersand furthermore parallel to the longitudinal axis of the radial arm 90.The diffusers are mounted to and retained in position by an air inletconduit, i.e. manifold 95B, which communicates with the air conveyingchannel 96 defined within the radial arm 90. As shown, the diffusers 92Bmay be dimensioned to extend from proximate the central shaft 15 outwardto proximate the tank sidewall 14.

As shown in FIG. 9, the rotating arm 90 typically rotates in acounter-clockwise direction (shown by arrows 109) about the centrallongitudinal axis 24 of shaft 22. Given this rotation, slurry flows in adirection generally shown by arrows 113 along the face of the diffusermembrane 93, thereby accomplishing the scouring function.

The pores of the diffuser are dimensioned such that in operation thediffusers 92 preferably produce air bubbles having a mean diameter lessthan approximately 4.5 millimeters.

Each diffuser frame 94 may include a solid back wall and a plurality ofupstanding sidewalls positioned on the back wall 108 and extendingoutwardly therefrom to form an open-box-like arrangement. Alternatively,opposing sides of the diffuser structure may be fitted with the porousflexible membrane.

The diffuser 92 includes a permeable replaceable membrane 93 which isfitted over the open end of each of the frames. The membrane issealingly adhered to the sidewalls whereby the frame 94 is sealed andrendered air-tight. Membrane 93 defines a plurality of pores or orificestherethrough.

As air is directed under pressure into the frame 94, those portions ofthe membrane which are not adhered to the back plate or sidewalls tendto bow outward. This bowing action enlarges the fabric as well as theair receiving chamber defined within the frame 94.

The following table lists a few of the preferred materials together withtest results obtained from utilizing those materials as a membrane inactual operation within a slurry and bacteria filled reactor vessel.

                  TABLE 1    ______________________________________                                   Reactor                                   Dissolved                Required Average   Oxygen  Useful                Air Rate Pressure  Conc.   Life    Diffuser Type                M.sup.3 /hr.                         Drop, Bar Mg/L    Days    ______________________________________    Elastox Perfo-                .33      .25         1-1.2 >60    rated Rubber    Tyvek 1042  .12      .65         3-3.5 10    Versapore 0.25                .13      .65         3-3.5  7    micron pore size    Porex sintered,                .65      .55       .5       8    porous plastic    Wilfley-Weber                .25      .55       2-3     20    Porous Ceramic    15 micron pore size    Wilfley-Weber                .15      1           3-3.5 20    Porous Ceramic    6 micron pore size    Polypropylene Felt,                .25      .55       2-3     10    (Filter Media)    Polypropylene                .25      .55       2-3     30    Felt (Silicone    Treated)    ______________________________________

The "useful life" is defined as that period of time which elapsed fromthe initiation of the test until the membrane was rendered ineffectivedue to clogging or damage incident to infestation by the bacteria.

The table indicates the rate of air required to be diffused through themembrane in order to achieve an O2 transfer rate of 200-300 mg/l/hr. tosupply an equivalent uptake rate by the bacteria. The O₂ uptake rate isa measurement commonly used in biological waste treatment. It reflectsthe rate of utilization of the oxygen within the slurry of the reactorvessel and is therefore a direct measure of biological activity. Itshould be understood that the oxygen transfer rate takes intoconsideration the form, i.e., bubble size, of the oxygen beingintroduced into the slurry. For example, a given quantity or mass ofoxygen may be introduced into the slurry in either the form of largebubbles or in the form of fine bubbles. The oxygen in fine bubbles ismore rapidly dissolved in the aqueous medium from which it can beassimilated by the bacteria. Therefore, notwithstanding the fact thatequal quantities of air would be introduced into the slurry in bothmethods, the O₂ transfer rate would not necessarily be equal for thelarge and fine bubble methods. Instead, the fine bubble method wouldhave a higher O₂ transfer rate, with more oxygen being supplied to theslurry in an assimilatable form from the same volume of air. Therefore,a larger portion of that O₂ could be assimilated by the bacteria beforethe bubbles reached the surface of the slurry and were discharged intothe environment. With reference to Table 1, all the diffusersillustrated, except the Porex sintered porous plastic, produced bubblesfiner than 4.5 mm in diameter.

As is deducible from a comparison of the data in Table 1, a diffusermembrane which produces bubbles having mean diameters greater than 4.5mm fail to produce a dissolved oxygen transfer rate comparable to thosemembrane producing bubbles having a diameter less than 4.5 mm.

The oxygen level in each reactor was monitored continuously with a YSIprobe as manufactured by the Yellow Springs Instrument Company. Theprocedure in determining the "oxygen uptake rate" was performed onsamples withdrawn from the reactor containing the diffuser. Theprocedure consisted of saturating the sample with oxygen in a speciallydesigned agitated vessel containing an O₂ probe, removing the O₂ source,capping the container and recording the rate of decrease in the O₂concentration. This value measured in mg/L/hr. is the take-up rate.

The support shaft 22 may also function as a drive shaft for purposes ofrotating the plurality of radially extending arm-like members 90 whichare positioned on that shaft 22 proximate the region near the bottom 17of the tank 14.

Positioned on the upper region of tank 14 is a bridge support 112 whichextends essentially across the diameter 21 of tank 14. In the centralregion of bridge 112 proximate the longitudinal axis 26 of tank 14, thebridge 112 includes an aperture which is adapted to receive the supportshaft 22 and permit the rotation of that shaft within the aperture. Apower transmission means 116 may be mechanically connected with theportion of shaft 22 which extends above bridge 112. This transmissionmeans 116 operates to rotate shaft 22 about its axis 26 and furthereffects a rotation of arms 90 and riffle tubes 70.

Fitted on the lower surface of each radial arm 90 may be a plurality ofrake-like extensions 117. These rake-like extensions 117 are adapted toeffect a squeegee-like action, i.e. scrape against the tank bottom 17,and thereby collect solids or particulates which have been deposited onsurface 118 and direct them to a central collecting location proximatethe end 61 of shaft 22. Rake-like extensions 117 may include a pluralityof planar panels, each panel having a respective longitudinal axis 119.As shown, each longitudinal axis 119 is oriented with respect to axis107 of the respective arm-like member 90 at a counter-clockwise rotationangle 120. Angle 120 may vary between approximately 45° to approximately90°. The critical aspect of the extensions 117 orientation is itscapacity to direct solids which have become deposited on the bottom 17or floor of the reactor vessel 13, to a common collection location.

The radially extending arms 90 may be mounted on shaft 22 to bevertically slidable along that shaft. In one construction, the arms 90and supports 91 are mounted to a tubular cylindrical sleeve 121 which isslidably positioned on the exterior of shaft 22. Sleeve 121 is maderotatable with shaft 22 by means of a releaseable key lock system whichlinks shaft 22 with sleeve 121. The slidability of sleeve 121 isenhanced by an elevational control system 122 which permits the operatorselectively to raise or lower the arms 90 at will. This control system122 may include a plurality of cables 123 which are mounted to eitherthe arms 90 or alternately to the cylindrical sleeve 121 whichinterconnects the various arms 90. The cables 123 extend vertically to awinch 125 or other means adapted to raise the cables 123 and effect acorresponding raising of the arms 90. The elevational control system 122is useful in freeing the arm 90/rake extension 117 assembly when thatassembly becomes mired in sediment collected on the bottom surface 17 ofthe tank 14. Further, the system 122 permits the operator to service thearms 90 without having to empty the tank 14.

As shown in FIG. 6, an internal filter 130 is positioned within theslurry 16 contained within tank 14. The filter 130 is adapted to drawliquid either continuously or semi-continuously from the slurry 16outward and into a conventional cloudy port filtrate receiver 132. Theinternal filter 130 and filtrate receivers 132 and 136 function toseparate clear liquor containing soluble metabolic by-products, e.g.,sulphuric acid and salts, from unoxidized and partially oxidized solids.

Internal filter 130 includes a porous medium having pores dimensioned tofilter solids from fluid. Owing to the relative size of the solidsvis-a-vis the medium pore size, a filter 130 initially permits somesolids to be introduced into filtrate conduit 134. The cloudy portfiltrate receiver 132 functions to retain these solids and reintroducethem into tank 14 along conduit 135. Upon the medium being sufficientlycoated with particulates, the operative medium pore size is reducedsufficiently that the enhanced filter effectively screens out solidsfrom the slurry liquid. As the filter 130 begins its enhanced operation,the liquid in filtrate conduit 134 is routed to a secondary filtratereceiver 136. Liquid or liquor which passes through this second receiver136 is thereafter discarded. The volume of liquor discarded is replacedby introducing water along conduit 137. This water serves to backwashfilter 130, removing the coating of solids which has collected thereon.

A second separation system 140 is shown in FIG. 7. The system 140includes means of removing a volume of slurry 16 continuously orsemi-continuously from the tank 14. The slurry 16 is then diluted by theaddition of water from conduit 142. Preferably, a flocculant is added tothe water or alternately the water/slurry mixture.

The slurry/water/flocculant is agitated to produce a rapid settlingfloc. The mixture is placed in a settling chamber 143 for at mostapproximately 10-15 minutes. During this time interval, the flocculatedparticles produced by the action of the flocculant settle out of themixture. The settled pulp which may include unoxidized, oxidized andpartially oxidized solids is then returned to the tank 14 along conduit144. The liquor or liquid portion of the composition is drawn offthrough an overflow arrangement and thereafter directed to wastedisposal or other treatment along conduit 147.

FIG. 5 illustrates a preferred system which operates to not only effecta solids-liquid separation but further operates to effect a separationof non-oxidized solids from partially oxidized solids. As shown, slurry16 is drawn either continuously or semi-continuously from tank 14through a conduit 148. Water is added to the slurry through conduit 149.The water/slurry mixture is then directed to cyclone 150. The cyclonefunctions to separate the relatively light non-oxidized solids from theheavier, partially oxidized solids which make up the remainingcomponents of the water/slurry mixture. The principles operative incyclone separation are well appreciated in the art. The non-oxidizedsolids are thereafter returned to tank 14 for purposes of processing.The liquid/partially oxidized solids mixture is then mixed with aflocculant as indicated by the block designated generally 154. Theflocculant/liquid/partially oxidized solids mixture is then directed toa sedimentation device 155 wherein the liquid is substantially separatedfrom the partially oxidized solids by a sedimentation process similar tothat shown in FIG. 7. The liquid is directed along conduit 156 totreatment or waste disposal.

The separated partially oxidized solids are channeled along conduit 157to a second bioreactor vessel 158 which operationally parallels that ofvessel 13. Vessel 158 includes a tank 163 adapted for retaining a slurrycomposed of metal-ladened solids, liquid, bacteria capable of oxidizingsulfide material, e.g., thiobacillus ferrooxidans and thiobacillusthiooxidans, nutrients such as oxygen and carbon dioxide.

Vessel 158 includes a separation system 159 for separating solids frommetabolic product-ladened slurry liquid. As shown, slurry 161 is drawneither continuously or semi-continuously from tank 163 through a conduit165. Water is added to the slurry through conduit 167. The water/slurrymixture is then directed to cyclone 169. The cyclone functions toseparate the relatively light, non-oxidized solids from the heavierpartially oxidized solids which make up the remaining components of thewater/slurry mixture. The principles operative in this separationprocess are well appreciated in the art. The non-oxidized solids arethereafter returned to tank 163 for purposes of processing. Theliquid/partially oxidized mixture is then mixed with a flocculant asindicated by the block designated generally 170. Theflocculant/liquid/partially oxidized solids mixture is then directed toa sedimentation device 172 wherein the liquid is substantially separatedfrom the partially oxidized solids by a sedimentation process similar tothat shown in FIG. 7. The liquid is directed along conduit 174 totreatment or waste. The separated partially oxidized solids are directedalong conduit 175 to an external liquid/solid separation system or arerecycled back to reactor 158.

The design of the various separation systems is dictated by thenecessity of limiting the amount of time in which the solids (bothnon-oxidized and partially oxidized) are removed from the oxygen-richenvironment found within either the primary reactor vessel 14 orsecondary reactor vessel 158. The bacteria utilized in the instantinvention attach themselves to solid materials. When those materials areremoved from the vessel, provision of oxygen to the bacteria to maintaintheir activity rate is limited to that oxygen extant within theparticular volume of slurry removed, i.e., the removed slurry is nottypically provided with an independent supply of oxygen. Given thiscondition, the separating systems are configured to provide astreamlined arrangement for quickly removing the process delimitingsoluble metabolic by-products, found in the liquid portion of theslurry, to permit the reintroduction of the bacteria-laden solids backinto the oxygen-rich environment found within one of the reactor vessels13 or 158. In a preferred construction, the instant inventioncontemplates restricting the maintenance of the solids out of thereactor vessel environment to a time period of at most 10-15 minutes.

The process of the instant invention consists substantially of distinctsteps. The first step includes a grinding operation of the subjectmetal-bearing solids. Specifically, the solids are ground to apredetermined size to aid in extraction. The grinding operation servesto increase the surface area of the solids which are to be subjected tothe action of the bacteria. Further, the grinding of the solids aids inthe suspension of those solids within the liquid slurry. The actual sizeof the ground solids may be varied so as to correspond to the particularproperties of the material being processed.

Preferably, closed circuit grinding is utilized with a substantialrecycle ratio in order to provide a narrow sized range of solids. Thisgrinding operation enhances a subsequent separation in the bioleachingreaction and also makes filtration and washing of the final producteasier. Subsequent to the grinding operation, the ground solids areplaced within a storage thickener and concentrated to a dense slurry.This formation of a dense slurry permits the operator to eitherintermittently or continuously feed the solids into the bioreactorvessel. The slurry is introduced into the reactor vessel together with asufficient supply of bacteria, e.g., thiobacillus femoosidans orthiobacillus thiooxidans, and the requisite nutrients, oxygen and carbondioxide requisite for the action of the bacteria on the solids. Thenutrients may include nitrogen, phosphate, magnesium and potassium.

During the operation of the bioreactor vessel, compressed oxygen oroxygen containing air (hereinafter "oxygen") is continuously directeddownward through channels 86 whereupon reaching the lower regions ofshaft 22 the oxygen contained within the channels 86 is forced outwardlythrough radial arms 90 and subsequently ejected through diffusers 92. Asthe oxygen passes outward through the diffuser pores, small oxygenbubbles as opposed to larger aggregate bubbles are released into thefluid slurry 16.

Due to the rotation of the arms 90, the bubbles are distributed over awide, substantially horizontal planar area of the lower regions of thetank 14. This rotation, together with the small dimension of thediffuser pores effects a wide distribution of the very small oxygenbubbles. Further, the rotation aids in hindering any formation orcollision of bubbles, which collision may lead to the formation ofaggregate bubbles having a smaller surface to volume ratio than thatattendant a plurality of smaller oxygen bubbles.

The central drive shaft 22 is rotated at a relatively slow speed,preferably approximately four (4) revolutions per minute. Air bubblesrise through the slurry at an approximate rate of four to six inches persecond. The speed of the shaft 22 rotation is adjusted such that bubblesreleased by a first diffuser at a given location have risen out of thatlocation before the subsequent release of bubbles in that location by anadjacent second diffuser. Theoretically and ideally, each diffuserreleases bubbles over the complete surface area of its respective porousmembrane 1. Due to the rotation of the diffuser, a continuous andgenerally spiral-shaped helix configuration of bubbles, having a widthapproximately equal to that of the diffuser, is generated within theslurry and rises uniformly upward through the slurry. The speed of theshaft 22 is ideally adjusted whereby none of the helixes, as generatedby the respective diffusers, intersect one another. Thereby a pluralityof adjacently positioned helixes composed of bubbles rise uniformlythrough the body of slurry.

Naturally, given the turbulence and non-homogeneity of the slurry, thisidealized bubble flow pattern does not occur in practice. Instead, therotation speed may be adjusted to approximate the ideal flow pattern soas to optimize the dispersion of air bubbles within the body of theslurry.

The oxygen bubbles rise through the slurry 16 and thereby facilitate theassimilation of that oxygen by the bacteria residing within the tank.The effect of the small apertured diffuser pores in creating very finebubbles together with the rotative action of the rotary arms 90 servingto widely disperse those bubbles about the bottom of the tank creates acondition wherein a large portion of the oxygen in the bubbles isdissolved into the aqueous phase within the slurry 16. The small size ofthe bubbles acts to not only promote a rapid dissolution of thosebubbles into the aqueous phase, but further, enhances the probability ofdirect interfacial transfer of the oxygen to the bacteria. Thisinterfacial transfer contrasts with the conventional practice in whichoxygen is introduced into the slurry and agitated to encouragedissolution. Thereafter, under the conventional practice, upondissolution, the oxygen is assimilated by the bacteria. Under theinstant methodology, the vessel operator can introduce into the slurry alarge quantity of oxygen, a portion of which is adapted for directinterfacial assimilation by the bacteria. Further, the oxygen may besupplied in a quantity in excess of the needs of the bacteria, at anenergy consumption rate which is measurably smaller than theconventional approach. Indeed, under the prior practice, the high costof achieving an adequate oxygen supply for the bacteria resulted inprocesses wherein the supply was purposely limited to a quantity belowthat required for maximum bacterial activity due to energyconsiderations. Under the instant method, the energy consumption is soreduced that oxygen may be supplied in excess of the amounts requisitefor optimized bacterial activity, while maintaining energy costs withinan acceptable cost range.

The bubbles are introduced proximate the bottom of the tank. Due todifferences in specific gravity, the bubbles rise upwardly through theslurry. The slurry in contrast is being drawn downwardly as quantitiesof slurry proximate the bottom 129 of shaft 22 are being drawn into theinterior channel 34 of shaft 22 by the air-lift suspension system andthereafter directed upwardly within that channel 34. Eventually, theslurry is discharged over the slurry surface 27 through riffle tubes 70.The effect of this slurry flow creates a general downward movement ofthe slurry within the tank and exterior to the interior channel 34. Thisslurry flow serves to retard the upward movement of the oxygen bubbles.Further, this retardation increases the residence time of the bubbleswithin the slurry and thereby enhances the probability that the oxygenwill be dissolved within the slurry and utilized by the bacteria.

During the operation of the bioreactor vessel cold water is injectedinto the channel 49 of the shaft 22 and forced downwardly therein. Theexothermic nature of the reaction occurring within the tank 14 serves toheat the slurry 16 within the tank. The cold water being separated fromthe slurry by the wall of shaft 22 absorbs heat through that wall fromthe higher temperatured slurry 16 as it continues downwardly through thechannel 49 of the shaft 22. Subsequently, the warmed or heat-laden wateris drawn upwardly through channel 53 and upon reaching the upper regionsof shaft 22 the water is discharged or cooled in an external heatexchanger or cooling tower and recycled.

During the operation of the bioreactor vessel 13, air is directeddownward along the interior of pipe 57, eventually exiting through adiffuser or nozzle 59. Slurry 16 which is within channel 34 of shaft 22thereafter is driven upwardly by the motion of the air bubbles formed atthe tip of diffuser 59. The air bubble/slurry mixture rises upwardlythrough the interior channel 34 of shaft 22. The slurry 16 subsequentlyexits through riffle tubes 70 and is distributed over the surface 78 ofthe slurry 16.

During the operation of the reactor vessel 13 an internal filtrationsystem 130 operates continuously or semi-continuously to remove solubleproducts ladened solution from within the slurry mixture. These productsmay include sulphates, sulphuric acid and arsenic acid. As shown, afilter medium 130 serves initially to screen solid particulates fromentering a conduit system which is directed to a cloudy port filtrationsystem 132. The internal filter 130 is fitted with a backwash waterconduit 137 whereby water may be injected along conduit 137 and toreplace the solution removed as well as to discharge particulates whichhave collected on the internal filter medium 130.

Critical to a proper operation of the instant invention is the controlof the constituents and environment within the reactor vessel. Thefactors of special importance include temperature, the rate andmechanism of oxygen input, the ratio of biomass (bacteria) to suspendedsolids, the ratio of reacted solids to unreacted solids, theconcentration of soluble species generated as products or by-productsand the concentration of carbon dioxide and nutrients.

The preferred species of bacteria utilized in the instant process arethiobacillus ferrooxidans or thiobacillus thiooxidans which are moststable and exhibit the broadest set of enzymematic activity when theirambient temperature is maintained in approximately the 35-36 Celsiusrange, i.e., the mesophilic range. Upon the temperature rising aboveapproximately 46 Celsius, these particular species of bacteria areeither extinguished or their activity severely limited. In that thereaction effected within the reactor vessel is exothermic in nature,absent a withdrawal of the heat produced in reaction, the stability ofthe bacteria is sacrificed. Accordingly, the instant reactor vesselincludes a heat exchanger adapted to absorb heat produced within thevessel 13 and transfer that heat from the vessel to effect thereby anoptimized thermal condition for bacteria growth and activity.

The rate and mechanism of oxygen input into the vessel has beendiscussed above. Due to the input of oxygen into the slurry in the formof widely dispersed small bubbles (i.e., having mean diameters less thanapproximately 45 mm.) a high surface to volume ratio of oxygen isobtained. The minute size of the bubbles effects an increased ratio ofdissolution or transition of the oxygen directly into the water.Further, the bubble size results in an enhanced quantity ofoxygen-ladened bubbles which permit interfacial transfer of the oxygento the bacteria. Owing to the density difference between the oxygenbubbles and the slurry, the bubbles have a limited residence time withinthe slurry before rising to the surface of the slurry and discharginginto the environment. The present invention involves a means of makingthe oxygen readily assimilatable upon its input into the slurry.

Resultingly, the oxygen is in a useful form throughout its ascensiontime through the slurry. Indeed, in tests conducted with a prototype ofthe vessel, oxygen uptake rates in the range of 500 milligrams per literper hour were obtained at an oxygen transfer efficiency greater than60%. Efficiency is defined for this instance as the amount of oxygenabsorbed by the bacteria divided by the initial amount of oxygenintroduced into the vessel.

The rotation of the arms effects a dispersion of the bubbles about asubstantially horizontal plane within the vessel. The arms are thereforerelatively slow in rotation in comparison to the typical tip speed ofturbines used in the conventional methodology.

Resultingly, the arms avoid cavitation effects thereby preserving thehigh surface/volume ratio of the bubbles. Further, the relative slow armrotation minimizes both the turbulence within the slurry and the energyrequirements requisite to operate the vessel.

In normal operation, the oxygen input rate is maintained at a constantrate. This rate is of sufficient magnitude to exceed the needs of thebacteria resident within the vessel. This approach contrasts with theconventional method wherein, due to the energy expense, the oxygensupply may typically be held to a quantity below the requisite level foroptimum bacterial activity.

The most important criteria attending the optimum operation of theinstant process is the maintenance of a high biomass to solids ratio.The biomass population may be limited by an inadequate supply of oxygen,carbon dioxide, nutrients, or alternately an excessive supply of solublemetabolic end products or by-products. Under the instant method thesupply of oxygen, carbon dioxide and nutrients are maintained at levelswhich exceed the demands of the bacteria population. The metabolic endproducts are selectively removed from the slurry during the vessel'soperation. These end products are constituted of two types: solubleconstituents and insoluble reacted solids. The soluble constituents areremoved by processing the slurry to effect a separation of suspendedsolids from the liquid liquor or medium. This separation is achieved bya continuously or semi-continuously operating, internal filter withinthe bioreactor. Alternatively, the separation may be achieved bysedimentation.

Flocculation of all of the suspended solids may be employed to enhancethe solids-liquid separation. Upon separation, the clarified liquor isremoved from the system while captured solids are advanced either to asecond reactor vessel or are returned to the first reactor vessel. Thebacteria typically adhere to the solid material. The separation ofsolids from liquid medium results in a minimum loss of bacteria from thevessel population in that the separated solids are subsequently andquickly returned to the first or second reactor vessel.

The solids removed may include reacted solids and unreacted solids. Thereacted solids are separated from the unreacted solids by the use ofselective separation in a cyclone, centrifuge or gravity settlingdevice. Unreacted solids are returned to the reactor vessel. Partiallyreacted solids are advanced to a secondary reactor for purposes offurther bacterial processing. The final reacted product is removed andmay be subjected to conventional leaching.

The emphasis of the instant process is the maintenance of the drivingforce of the reaction at a maximum. Due to the improvement in oxygensupply technology of the instant invention, the method presently resultsin the optimization of processing by providing a surplus of oxygen andother requisite nutriment, stabilizing the ambient temperature to anoptimal level and further removing reaction delimiting metabolicby-products. Further, this removal is continuous and operates tominimize the loss of bacteria which resulted under the conventionalmethod.

The embodiment illustrated in FIGS. 39 and 40 includes a grid-likesupport frame 100 which is stationarily mounted on the walls 14 of thetank and slidably secured to the central shaft 15. In this construction,the shaft 15 is made rotatable about its axis 24. The grid frame 100does not rotate with the shaft 15. Frame 100 includes a first pluralityof elongate conduits 1 arranged spaced apart and parallel to oneanother. A second plurality of the conduits 1 is also arranged spacedlyapart and parallel to one another. The first plurality of conduits isarranged perpendicular to the second plurality of conduits therebydefining the grid-like arrangement as shown. m The conduits 1 aremounted to one another at their points of interaction such that theinterior channels defined within the interior of each respective conduitcommunicates one with another. Diffusers 92A of the type illustrated inFIGS. 18 and 19 are shown installed at each intersection of a conduit101 of the first plurality of conduits and a conduit 101 of the secondplurality of conduits. Understandably, the configuration of thediffusers fitted on the grid-like frame support may be modified toinclude any desired configuration, e.g. horizontally oriented,quadrilaterally perimetered membrane diffusers, uprightly orientedplanar on curved face diffusers, uprightly oriented or horizontallyoriented tubular-shaped diffusers. The grid frame 100 is shown slidablyattached to shaft 15 by an annular collar 102.

In the embodiment illustrated in FIGS. 39 and 40, the radial arms 90 arefitted solely with rake-like extensions 117. The shaft 22 and radialarms 90 are rotatable in the embodiment similar to the structure shownin FIG. 1. Since the diffusers 92B in this construction are locationallyfixed, this construction relies mainly on the agitation created by theair bubbles rising through the slurry to retain the solids in the slurryin suspension. Further, little if any slurry flow over the diffusermembrane 93 occurs in this construction. Clogging of the diffusermembrane pores is alleviated by passing a quantity of air through thediffuser 92 at a pressure sufficient to dislodge any solids which mayhave clogged the diffuser pores. Since the membrane is fabricated from aflexible fabric, the fabric tends to expand under the higher airpressure loading similar to the action of a balloon. Those solids whichhave collected in the diffuser pores are resultingly forcefully ejectedoutward from the membrane surface.

In this embodiment, the operation of the rake-like extensions 177becomes more important than in the rotating diffuser embodiments, e.g.the embodiment of FIG. 1. Since the diffusers 92 do not rotate,concentrations of solids settling out of the slurry tend to build up onthe tank bottom 17 between adjacent pairs of diffusers 92. Absent somemeans of eliminating these concentrations, the possibility exists thatthese concentrations may build up until they actually cover over thestationary diffusers 92. Therefore, the action of the rotating arms 90with their attendant rake-like extensions 117 which sweep and scour thetank bottom 27 tend to eliminate concentrations of settled solids byoperating to lessen, if not eliminate the likelihood that solidconcentrations of sufficient size to hinder the diffuser operation willbe built up.

In those embodiments having an air lift suspension e.g. defined with thecenter shaft 22, the solids swept from the tank bottom are directed tothe center of the tank bottom 17 due to the orientation of the rake-likeextensions 117. The air lift within the center shaft 15 operates to pickup solids from proximate the center region of the tank bottom and liftthem above the surface level of the slurry in the tank. The solids arethen poured over and onto that slurry surface by arms 70 therebyredistributing the solids into the slurry.

It is to be understood that the embodiments of the invention hereindescribed are merely illustrative of the application of the principlesof the invention. Reference herein to details of the illustratedembodiment is not intended to limit the scope of the claims whichthemselves recite those features regarded as essential to the invention.

EXAMPLE

A continuous bioleaching pilot plant was operated on apyrite-arsenopyrite gold concentrate containing about one ounce of goldper ton, provided by a Canadian gold mine, to determine if this methodwould be viable for improving gold recovery by subsequent cyanidation.The test campaign lasted several months while the bacteria wereacclimated to this particular concentrate. Applicants utilized atwo-stage biological leaching circuit with cyclone separation betweenstages.

Applicants assembled a continuous bioleaching pilot plant in theirResearch Laboratory, using two 60 L. bioreactors similar to the unitillustrated schematically in FIG. 21. The unit is adapted such thatfinely ground concentrate may be fed continuously to the system,retained in an environment high in chemolithotrophic bacteriaconcentration for a period of days or weeks, and discharged as anoxidized product substantially reduced in iron, sulfur and arseniccontent. The procedure used was the following: feed slurry, afterregrinding of the concentrate, was introduced on an hourly basis from anagitated holding tank to the first stage reactor in which an activeculture of thiobacillus ferrooxidans bacteria and partially oxidizedsolids was maintained. Solution was withdrawn from the reactorcontinuously (though it could have been withdrawn semi-continuously,dependent upon the liquor density that was to be maintained) usingeither an internal filtration system 230 or sedimentation circuit 202.In the former, the solids were filtered and then back-washed into thereactor which, in effect, maintains both solids and bacteria within thesystem, while filtrate which is relatively free of biomass waswithdrawn. In the sedimentation mode, slurry was withdrawn from thebioreactor, diluted with wash water incoming through conduit 209,flocculated and thickened, and the thickener underflow returned to thebioreactor via conduit 211 while the liquor and some biomass overflowwas directed to waste via conduit 200. Both systems were utilized inthis study. Heat was removed from the reactor vessel by a conventionaltube heat exchanger 226.

Solids were advanced from the first stage to the second via ahydrocyclone, with the cyclone underflow being recycled to the reactorwhile the overflow solids were thickened and sent to the second unitthrough a continuous feeding system. Because of the capacity of thecyclone, it was necessary to limit the cycloning to about two times perweek, with approximately twenty percent (20%) of the reactor contentsprocessed at a time. Solids were removed from the second bioreactorthrough the same means.

The rationale for this approach is that it will maintain a highconcentration of sulfide- and arsenic-containing solids with thebioreactor while removing oxidized solids preferentially. It was foundthat cycloning could effect a separation between sulfide-bearingmaterial and solids which were low in sulfide, probably due to thedifference in density and the change in particle shape due tobio-oxidation. Since it is well known that a high concentration of foodmaterial (sulfide, arsenic) will result in a higher oxidation rate, thisapproach should minimize the reactor volume needed and maximize theoxygen transfer efficiency.

Two samples were received and tested; these samples analyzed as follows:

    ______________________________________               Fe %  As %      S %    SiO2 %    ______________________________________    First Sample 37.80   13.16     33.22                                        13.59    Second Sample                 37.81   8.39      28.00                                        19.38    ______________________________________

It had been reported that the gold is associated with the arsenopyrite,and that other biological process work had made possible a recovery bycyanidation of 86% of the gold. It seemed likely that this first workwould have concentrated on removing only the arsenopyrite (which isleached preferentially), and because of the relatively low recovery, itwas felt that it would be useful to reduce the pyrite concentration aswell. Thus, the tests were designed to solubilize as much of the pyriteand arsenopyrite as possible. It was recognized that some amount ofjarosite and ferric arsenate would form, and the jarosite would verylikely tie up some of the silver and make it difficult to extract bycyanidation. However, it was of special interest to determine if a2-stage system could extract both arsenopyrite and pyrite, since thebacteria seem to have a strong proclivity toward the former and poorextraction of pyrite could result. In the first tests, this appeared tobe the case, and the flowsheet was changed slightly so as to minimizethe effect of arsenic on the second stage. This was done by washing thematerial that was advanced from first stage to second stage in order toreduced the arsenic content of the liquor in the second stage. Since thebulk of the arsenic-bearing mineral was solubilized in the first stage,this effectively reduced the arsenic level background in the secondstage to that which is normal for ordinary pyrite systems.

Progress of the biological oxidation reactions was monitored bymeasuring the feed rate and the solids removal rate, as well as thesolution removal rate from each of the reactors, on a continuous basis.Solution and solids compositions within each of the reactors weremeasured twice a week, and materials which were removed were compositedand analyzed as required. The oxygen level in each reactor was monitoredcontinuously with a YSI probe 210 and attendant measurement meter 209.The "oxygen take-up rate," a measurement commonly used in biologicalwaste treatment, was performed daily on samples withdrawn from eachreactor.

The significant data are presented graphically in FIGS. 23-30 for thetwo bioreactors. It should be noted that "46" refers to the first stagereactor and "45" refers to the second stage, FIGS. 23-26 represent thefirst stage and FIGS. 27-30 the second. Dates are shown on the datapoints; on the time axis, only the days Monday-Friday are shown,although the calculated values include the weekends.

FIRST STAGE BIOREACTOR

With reference to FIG. 23, solution analyses include the liquor specificgravity, arsenic and iron concentrations. Normally, liquor specificgravities of 1.08-1.12 are tolerable, with iron levels as high as 60gpl. However, in view of the uncertainties about arsenic tolerance,liquor density was held down in order to keep the arsenic around 10 gpl,or lower. This was done by processing the required amount of slurrydaily through the continuous thickener, recycling the solids to thesystem.

FIG. 24, gas analysis, refers to the oxygen concentration in the reactorand the oxygen uptake rate of the slurry. Normally, when uptake ratesare low, the oxygen concentration approaches saturation, which is about5-6 mg/l under these conditions. As the uptake increases, the O₂concentration drops off, and in order to maintain it, additional air maybe applied to the diffusers. The apparent drop off in uptake rate duringthe last week of the campaign corresponded approximately to thetermination of the feed. As will be noted from the arsenic concentrationin the solids in the reactor, the actual arsenic level in the residue isfairly low, and it appeared that the principal reaction occurring inthis first stage was the oxidation of arsenopyrite.

The solid analyses are shown in FIG. 25, the percent solids in thereactor and the percent sulfide in the solids. It had been the intent ofthe program to raise the solids concentration up to about 30%, but,since this has to be done gradually in order to avoid overloading thesystem, a concentration above 20% was not achieved during this campaign.The percent sulfide in the solids generally was above 30%, even higherthan the feed material, due to the effect of recycling cyclone underflowin which the arsenopyrite had been partially solubilized. This confirmedthe earlier observations that the bacteria will preferentially oxidizearsenopyrite and will attack the pyrite only when the arsenic materialis depleted.

FIG. 26 shows the oxidation rate with respect to sulphur for this firststage reactor. It will be noted that this was higher during the firstpart of the campaign, dropping toward the end. This is apparently due tothe increased feed rate during the last two weeks, which was an averageof 1000 g/day of concentrate, compared to 600 g in the first 21/2 weeks.Again, this illustrates the preference of the bacteria for arsenic.

SECOND STAGE BIOREACTOR

The second stage unit was fed primarily with thickened cyclone overflowfrom the first stage reactor. In order to provide sufficient feedmaterial as necessary for proper biomass growth, since the cycloneunderflow was recycling most of the pyrite back to the first stage, aportion of the reactor slurry was diluted with water to remove thesoluble arsenic, thickened, and added to the feed system to the secondreactor.

FIG. 27, with data taken from Tables 3 and 4, presents the liquoranalysis for this unit. It will be noted that the arsenic level wasreduced significantly, while the liquor specific gravity was approachingthe target range of 1.1 which was desired.

The gas analysis (FIG. 28) for this reactor was typical of the behaviorwhich has been observed in other instances, with an inverse relationshipbetween O₂ concentration and take-up rate. The unusually widefluctuations were attributed to the lack of substrate (sulfide in thesolids) and as the feed rate was increased, the fluctuations werereduced. However, it apparent that the system was not receiving nearlyas much concentrate as it could have handled.

FIG. 29 presents the solids analysis for this reactor. The lack ofsufficient feed rate resulted in a much lower solids concentration thanwas desired, and it will be noted that the sulfide in the solids wasgenerally below 4%, contrasting with the 30% in the first stage andapproximately 11-25% in its feed material. The arsenic level in thesolids was usually around 1%, and it is believed that most of thisarsenic was present as precipitated ferric arsenate.

FIG. 30 illustrates the oxidation rate, and the increase in oxidationrate as the feed rate was increased confirms the earlier premise thatthis reactor was not receiving an adequate quantity of unoxidizedmaterial.

FIGS. 31 and 32 contain additional data collected on a daily basis onthe oxygen levels and oxygen take-up rates measured in the bioreactors.In FIG. 32, the periodic very rapid increases in take-up over a hourperiod are worth noting. This suggests that there is a certain level ofdormancy that occurs when insufficient food material is present.Maintaining an excess of food material should ensure continued oxidationrates much higher than those that were actually measured. It alsoindicates that the system can emerge quickly from dormant periods andreturn to full activity shortly after unoxidized material is added tothe system.

CYANIDATION TESTS

Four samples were withdrawn by cycloning of the material in the secondstage reactor, with the overflow thickened and washed prior tocyanidation. The cyanide strength was maintained at an average of about3 gpl NaCN and the total leaching time was 72 hours. It was stimatedthat the weight of the concentrate has been reduced by approximately72%, based on the relative increase in the insoluble content, and thegold concentration increased proportionally. The solutions that werewithdrawn from the bioreactors were analyzed by fire assaying and foundto contain virtually no gold. Thus, no loss of gold is expected duringbioleaching, and the subsequent cyanidation recovered about 97% of theamount present.

The bioleaching test work carried out on the concentrate sample provedthat the material can be processed with a continuous, 2-reactor systemin which arsenic is extracted in the first unit and sulfide mainly inthe second. The cyanidation of the oxidized product was very successful,with an average 97% recovery of gold values.

We claim:
 1. A reactor for use in processing metal-ladened solidsthrough use of a bioleaching process, said reactor comprising:acontainment means, having a base member, for containing a metal-bearingsolids slurry; a frame stationarily mounted within said containmentmeans having an open lattice configuration; an oxygen supply means forintroducing oxygen-containing gas into said solids slurry, said supplymeans including a plurality of porous, flexible membrane diffusersmounted horizontally and spacedly on said frame, each said porous,flexible membrane diffuser being adapted to receive a supply ofoxygen-containing gas and introducing said gas into said slurry in theform of bubbles; and a sweeping means, mounted within said containmentmeans below said frame for sweeping said base member of solids whichhave settled on said base member from said slurry.
 2. The reactor ofclaim 1 wherein said frame support is oriented horizontally andpositioned above said base member.
 3. The reactor of claim 1 whereineach said diffuser has a substantially planar diffusing face.
 4. Thereactor of claim 1 wherein each said diffuser is elongate and tubularshaped.
 5. The reactor of claim 1 wherein each said diffuser includes aplurality of elongate, tubular shaped members, each said member having aporous, flexible member exterior surface.
 6. A reactor for use inprocessing metal-ladened solids through use of a bioleaching process,said reactor comprising:a containment means, having a base member, forcontaining a metal-bearing solids slurry; a frame stationarily mountedwithin said containment means, above said base member, said frame havingan open lattice configuration; a plurality of flexible porous membranediffusers spacedly mounted on said frame, each said diffuser having ahorizontally oriented diffuser face for introducing oxygen-containinggas, in the form of bubbles, into said slurry; a supply means mounted onsaid diffusers for supplying oxygen-containing gas to said diffusers; anupright rotatable center shaft rotatably mounted within said containmentmeans; and a plurality of rake-fitted arms mounted on said center shaftto be rotatable therewith, said rakes being positioned below saidsupport frame and proximate said base member, wherein a rotation of saidcenter shaft effects a raking action by said rakes over said base memberto sweep solids which have settled on said base member from said solidsslurry.
 7. The reactor of claim 6 wherein said center shaft is fittedwith an air lift means, said rake-fitted arms being oriented to directsolids swept thereby into said air lift means whereby said solids areremoved from said base member via said air lift means and transportedabove said slurry and poured out on said slurry.
 8. The reactor of claim6 wherein said center shaft defines a hollow elongate channel thereinwhich communicates with said slurry proximate said base member, saidcenter shaft having a diffuser positioned within said channel forintroducing a quantity of air into slurry residing within said channel;said center shaft further including a plurality of arms mounted above asurface level of said slurry, said arms communicating with said channel,wherein slurry lifted through said center shaft may be directed throughsaid arms and poured through said arms onto said slurry surface.
 9. Areactor for use in processing metal-ladened solids through use of abioleaching process, said reactor comprising:a containment means, havinga base member, for containing a metal-bearing solids slurry; an uprightrotatable center shaft rotatably mounted within said containment means;a plurality of radial arms mounted on said center shaft, above said basemember; a frame support stationarily mounted within said containmentmeans elevationally above said radial arms, said frame having an openlattice configuration; a plurality of flexible porous membrane diffusersspacedly mounted on said frame, each said diffuser having a horizontallyoriented diffuser face for introducing oxygen-containing gas, in theform of bubbles, into said slurry; a supply means mounted on saiddiffusers for supplying oxygen-containing gas to said diffusers; and aplurality of rake-like members mounted on said radial arms to berotatable therewith, said rakes being positioned below said diffusersand proximate said base member, wherein a rotation of said center shafteffects a raking action by said rake members over said base member tosweep solids which have settled on said base member from said solidsslurry.
 10. The reactor of claim 9 wherein said center shaft is fittedwith an air lift means, said rake-fitted arms being oriented to directsolids swept thereby into said air lift means whereby said solids areremoved from said base member via said air lift means and transportedabove said slurry and poured out on said slurry.
 11. The reactor ofclaim 10 wherein said center shaft defines a hollow elongate channeltherein which communicates with said slurry proximate said base member,said center shaft having a diffuser positioned within said channel forintroducing a quantity of air into slurry residing within said channel;said center shaft further including a plurality of arms mounted above asurface level of said slurry, said arms communicating with said channel,wherein slurry lifted through said center shaft may be directed throughsaid arms and poured through said arms onto said slurry surface.